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IRON. Iron is the second most abundant mineral on earth and is an essential nutrient for nearly all organisms. Iron is necessary for many varied functions in mammals, including the synthesis of DNA, the generation of energy from macronutrients by aerobic respiration, and the transport and metabolism of oxygen. Iron is highly reactive and is potentially toxic at high levels of intake; therefore, its utilization and storage present a major challenge for biological systems. Cellular iron exists primarily in its reduced ferrous (Fe+2) and oxidized ferric (Fe+3) states, and conversion of the mineral between these states serves to catalyze many reactions. One example is Fenton's reaction, whereby hydrogen peroxide is converted to highly reactive hydroxyl radicals (.OH).

Both ferric iron and the hydroxyl radicals generated by free iron in this reaction directly damage tissues by randomly inducing DNA strand breaks and by oxidizing and thereby damaging cellular proteins, lipids, metabolic cofactors, and nucleic acids. Therefore, it is not surprising that most iron in the cell is bound or sequestered by proteins, so that the concentration of free iron is very low (usually less than 1 × 1018 moles per liter). Many ironbinding proteins are enzymes that harness and bring specificity to the reactive properties of iron, whereas other proteins store or transport iron (Table 1). Protein-bound iron can accept electrons during enzyme-catalyzed reactions, enable proteins to recognize and bind substrates, and assist in the formation of defined protein structures.

Dietary Forms and Factors Affecting Iron Requirements

The Recommended Daily Allowance (RDA) for iron is 8 milligrams per day for men and postmenopausal women and 18 milligrams per day for premenopausal women. Adult males contain about 4 grams of total body iron (50 milligrams per kilogram of body weight), whereas menstruating women contain 40 milligrams per kilogram of body weight. Full-term infants are born with sufficient

Representative proteins that bind iron
Protein Function
Transport and Storage Proteins
DMT1 Intestinal iron uptake
FP1 Intestinal iron export
Ferritin Iron storage
Ribonucleotide reductase Synthesis of DNA precursors
Cysteine dioxygenase Amino acid metabolism
Oxygen carriers

iron stores to meet metabolic demands for the first 4 months of life. Breast milk contains 0.2 mg iron/liter; breast-feeding infants receive about 0.27 milligrams per day.

There are two natural dietary forms of iron: (1) inorganic salts of ferric iron, and (2) iron bound to a cyclic carbon ring called heme in the form of hemoglobin and myoglobin in meat products. Inorganic iron is readily liberated from food in the acidic lumen of the stomach but is not absorbed well in the small intestine because of its poor solubility at physiological pH and because it is sequestered by many dietary components that hinder absorption, including phytates, polyphenols, calcium, and fiber. Therefore, only a small percentage of injected iron salts are actually absorbed into the body, thereby indicating that iron salts have a low bioavailability, or ability to be effectively absorbed. However, other low-molecular-weight dietary components bind inorganic iron and facilitate its absorption. These compounds, which include vitamin C and lactic acids, are commonly found in citrus and deciduous fruits and are known as metal chelators. In addition, an unidentified "meat factor" present in animal tissue also enhances the absorption of iron salts. Finally, heme iron has a much greater bioavailability than iron salts because fewer factors interfere with its absorption and it displays greater solubility in water. Hence, heme iron can account for up to 35 percent of absorbed iron in diets when accounting for only 10 percent of total dietary iron intake. In the United States, artificially fortified foods in the form of fortified grain products are a major source of dietary iron and account for nearly 50 percent of all iron consumed.

Iron absorption and transport from the intestinal lumen to the circulatory system is tightly regulated and complex. Enterocyte cells, which are responsible for the uptake and transport of nutrients from the intestinal mucosa, mediate the uptake and transport of iron to the plasma. These cells, once mature, function for only 48 to 72 hours before they are shed and excreted. The capacity of the mature enterocyte to transport inorganic iron is determined very early in its development and is inversely proportional to plasma iron status. The enterocyte iron transport protein, DMT1 (divalent metal transporter), facilitates iron uptake from the intestinal lumen into the enterocyte. DMT1 concentrations at the cell surface are increased when whole-body iron stores are depleted, which increases the rate of cellular iron accumulation into the enterocyte once it is matured. The induction of DMT1 protein synthesis results from increased DMT1 messenger RNA levels. During iron deficiency, the iron regulatory protein (IRP) binds to the 3' untranslated region of the DMT1 messenger RNA and increases its stability. Heme iron is transported into the enterocyte from the intestinal lumen by an unidentified heme iron receptor, and cellular enzymes in the enterocyte release iron from the heme ring. Iron is exported from the basolateral surface of the enterocyte to plasma by the iron transport protein ferroportin1 (Fp1). Fp1 is believed to assist in the direct transfer of iron to a soluble plasma iron transport protein called transferrin. Transferrin facilitates the delivery of two molecules of iron among the sites of absorption and storage and to all tissues and organs. The transferrin-iron complex enters the cell by binding to a specific protein, the transferrin receptor, which is present on the plasma membrane of all cells. Once transferrin binds to its receptor, the receptor-transferrin complex is engulfed by the cell, forming an internal vesicle called an endosome. Once in the cell, iron is released from transferrin by the acidification of the endosome, and the transferrin receptor is recycled to the cell surface where it can bind additional transferrin molecules.

Iron Physiology

Intestinal absorption is the primary mechanism that regulates whole body iron concentrations. There are no specific mechanisms to remove excess iron from mammals. Inorganic iron excretion is limited because of its low solubility in aqueous environments and therefore daily iron loss is minimal in the absence of blood loss. Fecal (from shed enterocytes and biliary heme products), urogenital, and integumental losses account for 4 mg/day of iron loss. Menstruation, blood donation, and pregnancy also can cause significant iron loss. Variations in iron status and requirements are influenced by individual genetic makeup as well as by differences in menstrual losses. The latter averages 0.6 mg/day but can greatly exceed that value in the individual, resulting in a need to absorb an additional 3 to 4 mg/day to maintain adequate iron status. An additional 4 to 5 mg/day of iron must be absorbed during pregnancy. States of rapid growth during childhood through adolescence also increase iron requirements.

Most absorbed iron is used by the bone marrow to make hemoglobin, an abundant protein that binds and distributes oxygen throughout the body. The remaining iron is distributed to other tissues where it is incorporated into iron-requiring proteins or stored. Nearly 70 percent of total body iron is present in red blood cells bound to hemoglobin. Another 15 percent is bound to metabolic enzymes and numerous other proteins, including muscle myoglobin, which transports oxygen to the mitochondria, and cytochromes, which act as electron carriers during respiration. The remaining iron is stored in the liver, spleen, and macrophages and can be distributed to other cells during states of dietary iron deficiency. The primary iron storage protein is ferritin, which is a hollow sphere comprised of 24 protein subunits. One ferritin molecule can store about 3,000 ferric iron molecules that can be mobilized readily when required. There are two types of ferritin subunits, heavy-chain and light-chain ferritin. Heavy-chain ferritin sequesters Fe+2 and oxidizes it to Fe+3; light-chain ferritin aids in the formation of the mineral iron core within the protein. Tissue, gender, hormones, and iron status can influence the ratio of heavy-chain and light-chain subunits that comprise a ferritin molecule, but the physiological significance of this ratio is not well understood.

Consequences of Altered Iron Status

Iron deficiency is the most common of all micronutrient deficiencies in the world, and the anemia that results affects an estimated 2 billion people. Dietary iron deficiency results in reduced iron stores in the liver, bone marrow, and spleen, followed by diminished erythropoiesis, which is the production of red blood cells, and anemia, and ultimately results in decreased activity of iron-dependent enzymes. Iron uptake in the intestine is responsive to total body stores such that iron-deficient individuals display increased iron absorption as described above. Clinical manifestations of iron deficiency include impaired endurance exercise due to an inability to deliver oxygen to tissues, microcytic anemia, glossitis, and blue scerra. Maternal iron deficiency during pregnancy is associated with several adverse outcomes for the newborn infant, including premature delivery, low birth weight, permanent cognitive deficits, developmental delay, and a wide range of behavioral disturbances. The onset of anemia and depletion of tissue iron concentrations occur concurrently, whereas the other negative consequences of iron deficiency occur after hemoglobin concentrations fall.

The tolerable upper level intake for iron for adults is 45 mg/day; intakes that exceed this level result in gastrointestinal distress. Dietary overload can occur, although it is uncommon, except in individuals with primary hereditary hemochromatosis, an iron-storage disease, which can result in up to fifty-fold increases in storage iron deposits. Hemochromatosis most commonly results from a common genetic mutation or genetic polymorphism in the HFE gene that is prevalent in populations of European descent but can also result from mutations in other iron-related proteins including a transferrin receptor. The HFE protein is involved in intestinal regulation of iron accumulation, but its precise biochemical function is unknown. This genetic disorder, if untreated by regular phlebotomy, results in liver cirrhosis, cadiomyopathy, arthritis, and cancer.

See also Gene Expression, Nutrient Regulation of; Nutrients; Nutrient Bioavailability .


Standing Committee on the Scientific Evaluation of Dietary

Reference Intakes, Food and Nutrition Board, Institute of Medicine. Washington, D.C.: National Academy Press, 2001. Dietary Reference Intakes for vitamin A, vitamin K, arsenic, boron, chromium, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium, and zinc.

Griffiths, William, and Timothy Cox. "Haemochromatosis:

Novel Gene Discovery and the Molecular Pathophysiology of Iron Metabolism." Human Molecular Genetics 9 (2000): 23772382.

Patrick J. Stover

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Iron is one of the most common elements on earth. Nearly every construction of man contains at least a little iron. It is also one of the oldest metals and was first fashioned into useful and ornamental objects at least 3,500 years ago.

Pure iron is a soft, grayish-white metal. Although iron is a common element, pure iron is almost never found in nature. The only pure iron known to exist naturally comes from fallen meteorites. Most iron is found in minerals formed by the combination of iron with other elements. Iron oxides are the most common. Those minerals near the surface of the earth that have the highest iron content are known as iron ores and are mined commercially.

Iron ore is converted into various types of iron through several processes. The most common process is the use of a blast furnace to produce pig iron which is about 92-94% iron and 3-5% carbon with smaller amounts of other elements. Pig iron has only limited uses, and most of this iron goes on to a steel mill where it is converted into various steel alloys by further reducing the carbon content and adding other elements such as manganese and nickel to give the steel specific properties.


Historians believe that the Egyptians were the first people to work with small amounts of iron, some five or six thousand years ago. The metal they used was apparently extracted from meteorites. Evidence of what is believed to be the first example of iron mining and smelting points to the ancient Hittite culture in what is now Turkey. Because iron was a far superior material for the manufacture of weapons and tools than any other known metal, its production was a closely guarded secret. However, the basic technique was simple, and the use of iron gradually spread. As useful as it was compared to other materials, iron had disadvantages. The quality of the tools made from it was highly variable, depending on the region from which the iron ore was taken and the method used to extract the iron. The chemical nature of the changes taking place during the extraction were not understood; in particular, the importance of carbon to the metal's hardness. Practices varied widely in different parts of the world. There is evidence, for example, that the Chinese were able to melt and cast iron implements very early, and that the Japanese produced amazing results with steel in small amounts, as evidenced by heirloom swords dating back centuries. Similar breakthroughs were made in the Middle East and India, but the processes never emerged into the rest of the world. For centuries the Europeans lacked methods for heating iron to the melting point at all. To produce iron, they slowly burned iron ore with wood in a clay-lined oven. The iron separated from the surrounding rock but never quite melted. Instead, it formed a crusty slag which was removed by hammering. This repeated heating and hammering process mixed oxygen with the iron oxide to produce iron, and removed the carbon from the metal. The result was nearly pure iron, easily shaped with hammers and tongs but too soft to take and keep a good edge. Because the metal was shaped, or wrought, by hammering, it came to be called wrought iron.

Tools and weapons brought back to Europe from the East were made of an iron that had been melted and cast into shape. Retaining more carbon, cast iron is harder than wrought iron and will hold a cutting edge. However, it is also more brittle than wrought iron. The European iron workers knew the Easterners had better iron, but not the processes involved in fashioning stronger iron products. Entire nations launched efforts to discover the process.

The first known European breakthrough in the production of cast iron, which led quickly to the first practical steel, did not come until 1740. In that year, Benjamin Huntsman took out a patent for the melting of material for the production of steel springs to be used in clockmaking. Over the next 20 years or so, the procedure became more widely adopted. Huntsman used a blast furnace to melt wrought iron in a clay crucible. He then added carefully measured amounts of pure charcoal to the melted metal. The resulting alloy was both strong and flexible when cast into springs. Since Huntsman was originally interested only in making better clocks, his crucible steel led directly to the development of nautical chronometers, which, in turn, made global navigation possible by allowing mariners to precisely determine their east/west position. The fact that he had also invented modern metallurgy was a side-effect which he apparently failed to notice.

Raw Materials

The raw materials used to produce pig iron in a blast furnace are iron ore, coke, sinter, and limestone. Iron ores are mainly iron oxides and include magnetite, hematite, limonite, and many other rocks. The iron content of these ores ranges from 70% down to 20% or less. Coke is a substance made by heating coal until it becomes almost pure carbon. Sinter is made of lesser grade, finely divided iron ore which, is roasted with coke and lime to remove a large amount of the impurities in the ore. Limestone occurs naturally and is a source of calcium carbonate.

Other metals are sometimes mixed with iron in the production of various forms of steel, such as chromium, nickel, manganese, molybdenum, and tungsten.

The Ore Extraction and Refining Process

Before iron ore can be used in a blast furnace, it must be extracted from the ground and partially refined to remove most of the impurities.

Historically, iron was produced by the hot-blast method, or later, the anthracite furnace. Either way, the fundamental activity in iron making involved a worker stirring small batches of pig iron and cinder until the iron separated from the slag. Called "puddling," this was highly skilled work, but was also hot, strenuous, and dangerous. It required a lot of experience as well as a hearty constitution. Puddlers were proud, independent, and highly paid.

Puddlers founded the first trade union in the iron and steel industry, the Sons of Vulcan, in Pittsburgh in 1858. In 1876, this union merged with three other labor organizations to form the Amalgamated Association of Iron and Steel Workers. This was the union that Andrew Carnegie defeated in the Homestead Strike of 1892, leaving the union in shambles and the industry essentially unorganized until the 1930s.

William S. Pretzer


  • 1 Much of the world's iron ore is extracted through open pit mining in which the surface of the ground is removed by heavy machines, often over a very large area, to expose the ore beneath. In cases where it is not economical to remove the surface, shafts are dug into the earth, with side tunnels to follow the layer of ore.


  • 2 The mined ore is crushed and sorted. The best grades of ore contain over 60% iron. Lesser grades are treated, or refined, to remove various contaminants before the ore is shipped to the blast furnace. Collectively, these refining methods are called beneficiation and include further crushing, washing with water to float sand and clay away, magnetic separation, pelletizing, and sintering. As more of the world's known supply of high iron content ore is depleted, these refining techniques have become increasingly important.
  • 3 The refined ore is then loaded on trains or ships and transported to the blast furnace site.

The Manufacturing

Charging the blast furnace

  • 1 After processing, the ore is blended with other ore and goes to the blast furnace. A blast furnace is a tower-shaped structure, made of steel, and lined with refractory, or heat-resistant bricks. The mixture of raw material, or charge, enters at the top of the blast furnace. At the bottom of the furnace, very hot air is blown, or blasted, in through nozzles called tuye'res. The coke burns in the presence of the hot air. The oxygen in the air reacts with the carbon in the coke to form carbon monoxide. The carbon monoxide then reacts with the iron ore to form carbon dioxide and pure iron.

Separating the iron from the slag

  • 2 The melted iron sinks to the bottom of the furnace. The limestone combines with the rock and other impurities in the ore to form a slag which is lighter than the iron and floats on top. As the volume of the charge is reduced, more is continually added at the top of the furnace. The iron and slag are drawn off separately from the bottom of the furnace. The melted iron might go to a further alloying process, or might be cast into ingots called pigs. The slag is carried away for disposal.

Treating the gases

  • 3 The hot gases produced in the chemical reactions are drawn off at the top and routed to a gas cleaning plant where they are cleaned, or scrubbed, and sent back into the furnace; the remaining carbon monoxide, in particular, is useful to the chemical reactions going on within the furnace.

    A blast furnace normally runs day and night for several years. Eventually the brick lining begins to crumble, and the furnace is then shut down for maintenance.

Quality Control

The blast furnace operation is highly instrumented and is monitored continuously. Times and temperatures are checked and recorded. The chemical content of the iron ores received from the various mines are checked, and the ore is blended with other iron ore to achieve the desired charge. Samples are taken from each pour and checked for chemical content and mechanical properties such as strength and hardness.


There are a great many possible environmental effects from the iron industry. The first and most obvious is the process of open pit mining. Huge tracts of land are stripped to bare rock. Today, depleted mining sites are commonly used as landfills, then covered over and landscaped. Some of these landfills themselves become environmental problems, since in the recent past, some were used for the disposal of highly toxic substances which leached into soil and water.

The process of extracting iron from ore produces great quantities of poisonous and corrosive gases. In practice, these gases are scrubbed and recycled. Inevitably, however, some small amounts of toxic gases escape to the atmosphere.

A byproduct of iron purification is slag, which is produced in huge amounts. This material is largely inert, but must still be disposed of in landfills.

Ironmaking uses up huge amounts of coal. The coal is not used directly, but is first reduced to coke which consists of almost pure carbon. The many chemical byproducts of coking are almost all toxic, but they are also commercially useful. These products include ammonia, which is used in a vast number of products; phenol, which is used to make plastics, cutting oils, and antiseptics; cresols, which go into herbicides, pesticides, pharmaceuticals, and photographic chemicals; and toluene, which is an ingredient in many complex chemical products such as solvents and explosives.

Scrap iron and steelin the form of old cars, appliances and even entire steel-girdered buildingsare also an environmental concern. Most of this material is recycled, however, since steel scrap is an essential resource in steelmaking. Scrap which isn't recycled eventually turns into iron oxide, or rust, and returns to the ground.

The Future

On the surface, the future of iron productionespecially in the United Statesappears troubled. Reserves of high-quality ore have become considerably depleted in areas where it can be economically extracted. Many long-time steel mills have closed.

However, these appearances are deceiving. New ore-enrichment techniques have made the use of lower-grade ore much more attractive, and there is a vast supply of that ore. Many steel plants have closed in recent decades, but this is largely because fewer are needed. The efficiency of blast furnaces alone has improved remarkably. At the beginning of this century, the largest blast furnace in the United States produced 644 tons of pig iron a day. It is believed that soon the possible production of a single furnace will reach 4,000 tons per day. Since many of these more modern plants have been built overseas, it has actually become more economical in some cases to ship steel across the ocean than to produce it in older U.S. plants.

Where To Learn More


Lambert, Mark. Spotlight on Iron and Steel. Rourke Enterprises, 1988.

Hartley, Edward N. Iron and Steel Works of the World. International Publication, 1987.

Lewis, W. David. Iron and Steel in America. Hagley Museum, 1986.

Walker, R. D. Modern Ironmaking Methods. Gower Publication, 1986.

Joel Simon

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Iron (revised)


Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.


The period in human history beginning in about 1200 B.C. is called the Iron Age. It was at about this time that humans first learned how to use iron metal. But in some ways, one could refer to the current era as the New Iron Age. Iron is probably the most widely used and most important metal today. No other metal is available to replace iron in all its many applications.

Iron is a transition metal. The transition metals are the elements that make up Groups 3 through 12 in the periodic table. The periodic table is a chart that shows how elements are related to one another. The transition metals are typical metals in that they tend to be bright, shiny, silvery solids. They all tend to conduct heat and electricity well. And they usually have high melting points.




Group 8 (VIIIB)
Transition metal


Iron normally does not occur as a free element in the earth. In fact, iron was not of much value to humans until they learned how to free iron from its compounds. Once they could do that, humans were able to make tools, weapons, household implements, and other objects out of iron. This step marked the beginning of the Iron Age.

Iron is most valuable not as a pure metal, but in alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals. The best known and most widely used alloy of iron is steel. Steel contains iron and at least one other element. Today, specialized steels of all kinds are available for many different applications.

Discovery and naming

Ancient Egyptians had learned how to use iron before the First Dynasty, which began in about 3400 B.C. The Egyptians probably found the iron in meteorites. Meteorites are chunks of rock and metal that fall from the sky. Some meteorites are very rich in iron. The Egyptians made tools and jewelry out of iron.

Iron is probably the most widely used and most important metal today.

Iron was also known to early Asian civilizations. In Delhi, India, for example, a pillar made out of iron built in A.D. 415 still stands. It weighs 6.5 metric tons and remains in good condition after nearly 1,600 years.

Early Chinese civilizations also knew about iron. Workers learned to produce iron as early as 200 B.C. A number of iron objects, including cannons, remain from the Han period (202 B.C. to A.D. 221).

The Bible also includes many mentions of iron. For example, a long passage in the book of Job describes the mining of iron. Other passages tell about the processing of iron ore to obtain iron metal.

By the time of the Roman civilization, iron had become an essential metal. The historian Pliny (A.D. 23-79) described the role of iron in Rome:

It is by the aid of iron that we construct houses, cleave rocks, and perform so many other useful offices of life. But it is with iron also that wars, murders, and robberies are effected, and this, not only hand to hand, but from a distance even, by the aid of weapons and winged weapons, now launched from engines, now hurled by the human arm, and now furnished with feathery wings.

Even from the earliest days, humans probably seldom used iron in a pure form. It was difficult to make iron that was free of impurities, such as carbon (charcoal) and other metals. More important, however, it became obvious that iron with impurities was a stronger metal that iron without impurities.

It was not until 1786, however, that scientists learned what it was in steel that made it a more useful metal than iron. Three researchers, Gaspard Monge (1746-1818), C. A. Vandermonde, and Claude Louis Berthollet (1748-1822) solved the puzzLe. They found that a small amount of carbon mixed with iron produced a strong alloy. That alloy was steel. Today, the vast amount of iron used in so many applications is used in the form of steel, not pure iron.

Ancient Egyptians had learned how to use iron before the First Dynasty, which began in about 3400 B.C.

The chemical symbol for iron is Fe. That symbol comes from the Latin name for iron, ferrum.

Physical properties

Iron is a silvery-white or grayish metal. It is ductile and malleable. Ductile means capable of being drawn into thin wires. Malleable means capable of being hammered into thin sheets. It is one of only three naturally occurring magnetic elements. The other two are nickel and cobalt

Iron has a very high tensile strength. Tensile means it can be stretched without breaking. Iron is also very workable. Workability is the ability to bend, roll, hammer, cut, shape, form, and otherwise work with a metal to get it into a desired shape or thickness.

The melting point of pure iron is 1,536°C (2,797°F) and its boiling point is about 3,000°C (5,400°F). Its density is 7.87 grams per cubic centimeter. The melting point, boiling point, and other physical properties of steel alloys may be quite different from those of pure iron.

Chemical properties

Iron is a very active metal. It readily combines with oxygen in moist air. The product of this reaction, iron oxide (Fe2O3), is known as rust. Iron also reacts with very hot water and steam to produce hydrogen gas. It also dissolves in most acids and reacts with many other elements.

Occurrence in nature

Iron is the fourth most abundant element in the Earth's crust. Its abundance is estimated to be about 5 percent. Most scientists believe that the Earth's core consists largely of iron. Iron is also found in the Sun, asteroids, and stars outside the solar system.

The most common ores of iron are hematite, or ferric oxide (Fe2O3); limonite, or ferric oxide (Fe2O3); magnetite, or iron oxide (Fe3O4); and siderite, or iron carbonate (FeCO3). An increasingly important source of iron is taconite. Taconite is a mixture of hematite and silica (sand). It contains about 25 percent iron.

The largest iron resources in the world are in China, Russia, Brazil, Canada, Australia, and India. The largest producers of iron from ore in the world are China, Japan, the United States, Russia, Germany, and Brazil.


There are four naturally occurring isotopes of iron, iron-54, iron-56, iron-57, and iron-58. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Six radioactive isotopes of iron are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

Two radioactive isotopes of iron are used in medical and scientific research. They are iron-55 and iron-59. These isotopes are used primarily as tracers in studies on blood. A tracer is a radioactive isotope whose presence in a system can easily be detected. The isotope is injected into the system. Inside the system, the isotope gives off radiation. That radiation can be followed by detectors placed around the system. Iron-55 and iron-59 are used to study the way in which red blood cells develop in the body. These studies can be used to tell if a person's blood is healthy.


Iron goes through a number of stages between ore and final steel product. In the first stage, iron ore is heated with limestone and coke (pure carbon) in a blast furnace. A blast furnace is a very large oven in which the temperature may reach 1,500°C (2,700°F). In the blast furnace, coke removes oxygen from iron ore:

The limestone removes impurities in the iron ore.

Iron produced by this method is about 91 to 92 percent pure. The main impurity left is carbon from the coke used in the furnace. This form of iron is known as pig iron. Pig iron is generally too brittle (it breaks too easily) to be used in most products.

Most scientists believe that the Earth's core consists largely of iron.

A number of methods have been developed for purifying pig iron. A common method used today is called the basic oxygen process. In this process, pig iron is melted in a large oven. Then pure oxygen gas is blown through the molten pig iron. The oxygen burns off much of the carbon in the pig iron:

A small amount of carbon remains in the iron. The iron produced in this reaction is known as steel.

The term "steel" actually refers to a wide variety of products. The various forms of steel all contain iron and carbon. They also contain one or more other elements, such as silicon, titanium, vanadium, chromium, manganese, cobalt, nickel, zirconium, molybdenum, and tungsten. Two other steel-like products are cast iron and wrought iron. Cast iron is an alloy of iron, carbon, and silicon. Wrought iron contains iron and any one or more of many other elements. In general, however, wrought iron tends to contain very little carbon.


It would be impossible to list all uses of iron and steel products. In general, those products can be classified into categories: (1) automotive; (2) construction; (3) containers, packaging, and shipping; (4) machinery and industrial equipment; (5) rail transportation; (6) oil and gas industries; (7) electrical equipment; and (8) appliances and utensils. (For more information on specific kinds of steel alloys, see individual elements, such as titanium, vanadium, chromium, manganese, molybdenum, and tungsten.)


Some iron is made into compounds. The amount is very small compared to the amount used in steel and other iron alloys. Probably the fastest growing use of iron compounds is in water treatment systems. The terms ferric and ferrous refer to two different forms in which iron occurs in compounds. Some of the important iron compounds are:

The U.S. Recommended Daily Allowance (USRDA) for iron is 18 milligrams.

ferric acetate (Fe(C2H3O2)3): used in the dyeing of cloth

ferric ammonium oxalate(Fe(NH4)3(C2O4)4): blueprints

ferric arsenate (FeAsO4): insecticide

ferric chloride (FeCl3): water purification and sewage treatment systems; dyeing of cloth; coloring agent in paints; additive for animal feed; etching material for engraving, photography, and printed circuits

ferric chromate (Fe2(CrO4)3): yellow pigment (coloring) for paints and ceramics

ferric hydroxide (Fe(OH)3): brown pigment for coloring rubber; water purification systems

ferric phosphate (FePO4): fertilizer; additive for animal and human foods

ferrous acetate (Fe(C2H3O2)2): dyeing of fabrics and leather; wood preservative

ferrous gluconate (Fe(C6H11O7)2): dietary supplement in "iron pills"

ferrous oxalate (FeC2O4): yellow pigment for paints, plastics, glass, and ceramics; photographic developer

ferrous sulfate (FeSO4): water purification and sewage treatment systems; catalyst in production of ammonia; fertilizer; herbicide; additive for animal feed; wood preservative; additive to flour to increase iron levels

Health effects

Iron is of critical importance to plants, humans, and animals. It occurs in hemoglobin, a molecule that carries oxygen in the blood. Hemoglobin picks up oxygen in the lungs, and carries it to the cells. In the cells, oxygen is used to produce energy the body needs to survive, grow, and stay healthy.

The U.S. Recommended Daily Allowance (USRDA) for iron is 18 milligrams. The USRDA is the amount of an element that a person needs to stay healthy. Iron is available in a number of foods, including meat, eggs, and raisins.

An iron deficiency (lack of iron) can cause serious health problems in humans. For instance, hemoglobin molecules may not form in sufficient numbers. Or they may lose the ability to carry oxygen. If this occurs, a person develops a condition known as anemia. Anemia results in fatigue. Severe anemia can result in a lowered resistance to disease and an increase in heart and respiratory (breathing) problems. Some forms of anemia can even cause death.

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Iron is a mineral that the human body uses to produce the red blood cells (hemoglobin) that carry oxygen throughout the body. It is also stored in myoglobin, an oxygen-carrying protein in the muscles that fuels cell growth.

General use

Iron is abundant in red meats, vegetables, and other foods, and a well-balanced diet can usually provide an adequate supply of the mineral. But when there is insufficient iron from dietary sources, or as a result of blood loss in the body, the amount of hemoglobin in the bloodstream is reduced and oxygen cannot be efficiently transported to tissues and organs throughout the body. The resulting condition is known as iron-deficiency anemia , and is characterized by fatigue , shortness of breath, pale skin, concentration problems, dizziness , a weakened immune system, and energy loss.

Iron-deficiency anemia can be caused by a number of factors, including poor diet, heavy menstrual cycles, pregnancy , kidney disease, burns , and gastrointestinal disorders. Individuals with iron-deficiency anemia should always undergo a thorough evaluation by a physician to determine the cause.

Children two years old and under also need adequate iron in their diets to promote proper mental and physical development. Children under two who are not breastfed should eat iron-fortified formulas and cereals. Women who breastfeed need at least 15 mg of dietary or supplementary iron a day in order to pass along adequate amounts of the mineral to their child in breast milk. Parents should consult a pediatrician or other healthcare professional for guidance on iron supplementation in children.

It has been theorized that excess stored iron can lead to atherosclerosis and ischemic heart disease . Phlebotomy, or blood removal, has been used to reduce stored iron in patients with iron overload with some success. Iron chelation with drugs such as desferrioxamine (Desferal) that help patients excrete excess stores of iron can be helpful in treating iron overload caused by multiple blood transfusions.

Iron levels in the body are measured by both hemoglobin and serum ferritin blood tests.

Normal total hemoglobin levels are:

  • neonates: 17-22 g/dl
  • one week: 15-20 g/dl
  • one month: 11-15 g/dl
  • children: 11-13 g/dl
  • adult males: 14-18 g/dl (12.4-14.9 g/dl after age 50)
  • adult females: 12-16 g/dl (11.7-13.8 g/dl after menopause)

Normal serum ferritin levels are:

  • neonates: 25-200 ng/ml
  • one month: 200-600 ng/ml
  • two to five months: 50-200 ng/ml
  • six months to 15 years: 7-140 ng/ml
  • adult males: 20-300 ng/ml
  • adult females: 20-120 ng/ml


Iron can be found in a number of dietary sources, including:

  • pumpkin seeds
  • dried fruits (apricots)
  • lean meats (beef and liver)
  • fortified cereals
  • turkey (dark meat)
  • green vegetables (spinach, kale, and broccoli)
  • beans, peas, and lentils
  • enriched and whole grain breads
  • molasses
  • sea vegetables (blue-green algae and kelp)

Eating iron-rich foods in conjunction with foods rich in vitamin C (such as citrus fruits) and lactic acid (sauerkraut and yogurt) can increase absorption of dietary iron. Cooking food in cast-iron pots can also add to their iron content.

The recommended dietary allowances (RDA) of iron as outlined by the United States Department of Agriculture (USDA) are as follows:

  • Children 03: 6-10 mg/day
  • children 410: 10 mg/day
  • adolescentadult males: 10 mg/day
  • adolescentadult females: 10-15 mg/day
  • pregnant females: 30 mg/day
  • breastfeeding females: 15 mg/day

A number of herbal remedies contain iron, and can be useful as a natural supplement. The juice of the herb stinging nettle (Urtica dioica ) is rich in both iron and vitamin C (which is thought to promote the absorption of iron). It can be taken daily as a dietary supplement. Dandelion (Taraxacum officinale ), curled dock (Rumex crispus ), and parsley (Petroselinum crispum ) also have high iron content, and can be prepared in tea or syrup form.

In Chinese medicine, dang gui (dong quai ), or Angelica sinensis, the root of the angelica plant, is said to both stimulate the circulatory system and aid the digestive system. It can be administered as a decoction or tincture, and should be taken in conjunction with an iron-rich diet. Other Chinese remedies include foxglove root (Rehmannia glutinosa ), Korean ginseng (Panax ginseng ), and astragalus (Astragalus membranaceus ).

Ferrum phosphoricum (iron phosphate), is used in homeopathic medicine to treat anemia. The remedy is produced by mixing iron sulfate, phosphate, and sodium acetate, which is administered in a highly diluted form to the patient. Other homeopathic remedies for anemia include Natrum muriaticum, Chinchona officinalis, Cyclamen europaeum, Ferrum metallicum, and Manganum aceticum. As with all homeopathic remedies, the type of remedy prescribed for iron deficiency depends on the individual's overall symptom picture, mood, and temperament. Patients should speak with their homeopathic professional or physician, or healthcare professional before taking any of these remedies.

Iron is also available in a number of over-the-counter supplements (i.e., ferrous fumerate, ferrous sulfate, ferrous gluconate, iron dextran). Both heme iron and nonheme iron supplements are available. Heme iron is more efficiently absorbed by the body, but non-heme iron can also be effective if used in conjunction with vitamin C and other dietary sources of heme iron. Some multivitamins also contain supplementary iron. Ingesting excessive iron can be toxic, and may have long-term negative effects. For this reason, iron supplements should be taken only under the recommendation and supervision of a doctor.


Iron deficiency can be a sign of a more serious problem, such as internal bleeding. Anyone suffering from iron-deficiency anemia should always undergo a thorough evaluation by a healthcare professional to determine the cause.

Iron overdose in children can be fatal, and is a leading cause of poisoning in children. Children should never take supplements intended for adults, and should receive iron supplementation only under the guidance of a physician.

Individuals with chronic or acute health conditions, including kidney infection, alcoholism , liver disease, rheumatoid arthritis, asthma , heart disease, colitis, and stomach ulcer should consult a physician before taking herbal or pharmaceutical iron supplements.

If individuals taking homeopathic dilutions of ferrum phosphoricum experience worsening of their symptoms (known as a homeopathic aggravation), they should stop taking the remedy and contact their healthcare professional. A homeopathic aggravation can be an early indication that a remedy is working properly, but it can also be a sign that a different remedy is needed.

Patients diagnosed with hemochromatosis, a genetic condition in which the body absorbs too much iron and stores the excess in organs and tissues, should never take iron supplements.

Side effects

Taking herbal or pharmaceutical iron supplements on an empty stomach may cause nausea . Iron supplementation may cause hard, dark stools, and individuals who take iron frequently experience constipation . Patients who experience dark bowel movements accompanied by stomach pains should check with their doctor, as this can also indicate bleeding in the digestive tract.

Other reported side effects include stomach cramps and chest pain . These symptoms should be evaluated by a physician if they occur.

Some iron supplements, particularly those taken in liquid form, may stain the teeth. Taking these through a straw, or with a dropper placed towards the back of the throat, may be helpful in preventing staining. Toothpaste containing baking soda and/or hydrogen peroxide can be useful in removing iron stains from teeth.

Signs of iron overdose include severe vomiting , racing heart, bloody diarrhea , stomach cramps, bluish lips and fingernails, pale skin, and weakness. If overdose is suspected, the patient should contact poison control and/or seek emergency medical attention immediately.


Iron supplements may react with certain medications, including antacids, acetohydroxamic acid (Lithostat), dimercaprol, etidronate, fluoroquinolones. In addition, they can decrease the effectiveness of certain tetracyclines (antibiotics). Individuals taking these or any other medications should consult their healthcare professional before starting iron supplements.

Certain foods decrease the absorption of iron, including some soy-based foods, foods with large concentrations of calcium , and beverages containing caffeine and tannin (a substance found in black tea). These should not be taken within two hours of using an iron supplement. Some herbs also contain tannic acid, and should be avoided during treatment with iron supplements. These include allspice (Pimenta dioica ) and bayberry (Myrica cerifera, also called wax myrtle).

Individuals considering treatment with homeopathic remedies should also consult their healthcare professional about possible interactions with certain foods, beverages, prescription medications, aromatic compounds, and other environmental elementsfactors known in homeopathy as remedy antidotes that could counteract the efficacy of treatment for iron deficiency.



Medical Economics Company. PDR 2000 Physicians' Desk Reference. Montvale, NJ: Medical Economics Company, 1998.

Medical Economics Company. PDR for Herbal Medicines. Montvale, NJ: Medical Economics Company, 1998.

Ody, Penelope. The Complete Medicinal Herbal. New York: DK Publishing, 1993.


de Valk, B., and J.J.M. Marx. "Iron, Atherosclerosis, and Is-chemic Heart Disease." Archives of Internal Medicine 159(i14): 1542.

Paula Ford-Martin

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iron, metallic chemical element; symbol Fe [Lat. ferrum]; at. no. 26; at. wt. 55.845; m.p. about 1,535°C; b.p. about 2,750°C; sp. gr. 7.87 at 20°C; valence +2, +3, +4, or +6. Iron is biologically significant. Because iron is a component of hemoglobin, a red oxygen-carrying pigment of the red blood cells of vertebrates, iron compounds are important in nutrition; one cause of anemia is iron deficiency. For the history of the use of iron, see Iron Age.


Iron is a lustrous, ductile, malleable, silver-gray metal found in Group 8 of the periodic table. It is known to exist in four distinct crystalline forms (see allotropy). The most common is the α-form, which is stable below about 770°C, and has a body-centered cubic crystalline structure; it is often called ferrite. Iron is attracted by a magnet and is itself easily magnetized (see magnetism). It is a good conductor of heat and electricity. It displaces hydrogen from hydrochloric or dilute sulfuric acid, but becomes passive (loses its normal chemical activity) when treated with cold nitric acid.


Iron forms such compounds as oxides, hydroxides, halides, acetates, carbonates, sulfides, nitrates, sulfates, and a number of complex ions. It is chemically active and forms two major series of chemical compounds, the bivalent iron (II), or ferrous, compounds and the trivalent iron (III), or ferric, compounds. Ferrous sulfate heptahydrate, FeSO4·7H2O, sometimes called green vitriol, is a compound formed by the reaction of dilute sulfuric acid (formerly called oil of vitriol) with metallic iron; it is used in the manufacture of ink, in dyeing, and as a disinfectant. Ferric chloride hexahydrate, FeCl3·6H2O, is a yellow-brown crystalline compound used as a mordant in dyeing and as an etching compound. Ferric oxide, Fe2O3, is a reddish-brown powder used as a paint pigment and in abrasive rouges. Prussian blue, KFe2(CN)6, is a pigment containing the ferrocyanide complex ion. Iron rusts readily in moist air, forming a complex mixture of compounds that is mostly a ferrous-ferric oxide with the composition Fe3O4.

Natural Occurrence

Iron is an abundant element in the universe; it is found in many stars, including the sun. Iron is the fourth most abundant element in the earth's crust, of which it constitutes about 5% by weight, and is believed to be the major component of the earth's core. Iron is found distributed in the soil in low concentrations and is found dissolved in groundwaters and the ocean to a limited extent. It is rarely found uncombined in nature except in meteorites, but iron ores and minerals are abundant and widely distributed.

The principal ores of iron are hematite (ferric oxide, Fe2O3) and limonite (ferric oxide trihydrate, Fe2O3·3H2O). Other ores include siderite (ferrous carbonate, FeCO3), taconite (an iron silicate), and magnetite (ferrous-ferric oxide, Fe3O4), which often occurs as a white sand. Iron pyrite (iron disulfide, FeS2) is a crystalline gold-colored mineral known as fool's gold. Chromite is a chromium ore that contains iron. Lodestone is a form of magnetite that exhibits natural magnetic properties.

Production and Refining

Iron is produced in the United States chiefly from oxide ores. For many years rich hematite ores were produced by open-pit mining in the Mesabi Range near Lake Superior. However, these ores have been largely depleted, and iron is now produced from low-grade ores that are treated to improve their quality; this process is called beneficiation. Iron ores are refined in the blast furnace. The product of the blast furnace is called pig iron and contains about 4% carbon and small amounts of manganese, silicon, phosphorus, and sulfur. About 95% of this iron is processed further to make steel, often by the open-hearth process or the Bessemer process, but more recently in the United States and other countries by the basic oxygen process or by an electric arc furnace. The balance is cast in sand molds into blocks called pigs. It is further processed in iron foundries (see casting).

Cast Iron

Cast iron is made when pig iron is remelted in small cupola furnaces (similar to the blast furnace in design and operation) and poured into molds to make castings. It usually contains 2% to 6% carbon. Scrap iron or steel is often added to vary the composition. Cast iron is used extensively to make machine parts, engine cylinder blocks, stoves, pipes, steam radiators, and many other products. Gray cast iron, or gray iron, is produced when the iron in the mold is cooled slowly. Part of the carbon separates out in plates in the form of graphite but remains physically mixed in the iron. Gray iron is brittle but soft and easily machined. White cast iron, or white iron, which is harder and more brittle, is made by cooling the molten iron rapidly. The carbon remains distributed throughout the iron as cementite (iron carbide, Fe3C). A malleable cast iron can be made by annealing white iron castings in a special furnace. Some of the carbon separates from the cementite; it is much more finely divided than in gray iron. A ductile iron may be prepared by adding magnesium to the molten pig iron; when the iron is cast the carbon forms tiny spherical nodules around the magnesium. Ductile iron is strong, shock resistant, and easily machined.

Wrought Iron

Wrought iron is commercially purified iron. In the Aston process, pig iron is refined in a Bessemer converter and then poured into molten iron silicate slag. The resulting semisolid mass is passed between rollers that squeeze out most of the slag. The wrought iron has a fibrous structure with threads of slag running through it; it is tough, malleable, ductile, corrosion resistant, and melts only at high temperatures. It is used to make rivets, bolts, pipes, chains, and anchors, and is also used for ornamental ironwork.


See W. H. Dennis, Metallurgy of the Ferrous Metals (1963) and Foundations of Iron and Steel Metallurgy (1967).

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Iron is the fourth-most common element in Earth's crust , and the second-most common metal after aluminum . Its abundance is estimated to be about 5%. Sampling studies indicate that portions of Earth's core consist largely of iron, and the element is found commonly in the Sun , asteroids , and stars.

The chemical symbol for iron, Fe, comes from the Latin name for the element, ferrum. The most common ores of iron are hematite and limonite (both primarily ferric oxide; Fe2O3) and siderite iron carbonate (FeCO3). An increasingly important source of iron for commercial uses is taconite, a mixture of hematite and silica. Taconite contains about 25% iron. The largest iron resources in the world are found in China, Russia, Brazil, Canada, Australia , and India.

The traditional method for extracting pure iron from its ore is to heat the ore in a blast furnace with limestone and coke. The coke reacts with iron oxide to produce pure iron, while the limestone combines with impurities in the ore to form a slag that can then be removed from the furnace: 3C + 2Fe2O3 + heat 3CO2 + 4Fe.

Iron produced by this method is about 90% pure and is known as pig iron. Pig iron is generally too brittle to be used for most products and is further treated to convert it to wrought iron, cast iron, or steel. Wrought iron is an alloy of iron and any one of many different elements, while cast iron is an alloy of iron, carbon , and silicon . Steel is a generic term that applies to a very wide variety of alloys.

Iron is one of a handful of elements that have been known and used since the earliest periods of human history. In the period beginning about 1200 b.c. iron was so widely used for tools, ornaments, weapons, and other objects that historians and archaeologists have now named the period the Iron Age.

Iron is a silvery white or grayish metal that is ductile and malleable. It is one of only three naturally occurring magnetic elements, the other two being its neighbors in the periodic table : cobalt and nickel. Iron has a very high tensile strength and is very workable, capable of being bent, rolled, hammered, cut, shaped, formed, and otherwise worked into some desirable shape or thickness. Iron's melting point is 2,797°F (1,536°C) and its boiling point is about 5,400°F (3,000°C). Its density is 7.87 grams per cubic centimeter.

Iron is an active metal that combines readily with oxygen in moist air to form iron oxide (Fe2O3), commonly known as rust. Iron also reacts with very hot water and steam to produce hydrogen gas and with most acids and a number of other elements.

The number of commercial products made of iron and steel is very large indeed. The uses of these two materials can generally be classified into about eight large groups, including (1) automotive; (2) construction; (3) containers, packaging, and shipping; (4) machinery and industrial equipment; (5) rail transportation; (6) oil and gas industries; (7) electrical equipment; and (8) appliances and utensils.

A relatively small amount of iron is used to make compounds that have a large variety of applications, including dyeing of cloth, blueprinting, insecticides, water purification and sewage treatment, photography, additive for animal feed, fertilizer, manufacture of glass and ceramics, and wood preservative.

Iron is of critical important to plants, humans, and other animals. It occurs in hemoglobin, the molecule that carries oxygen in the blood. The U.S. Recommended Daily Allowance (USRDA) for iron is 18 mg (with some differences depending on age and sex) and it can be obtained from meats, eggs, raisins, and many other foods. Iron deficiency disorders, known as anemias, are not uncommon and can result in fatigue, reduced resistance to disease, an increase in respiratory and circulatory problems, and even death.

See also Chemical bonds and physical properties; Chemical elements; Earth, interior structure; Minerals

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iron often in figurative use to mean stern and unyielding.
iron age originally the Greek and Roman poets' name for the last and worst period of human history, succeeding the gold, silver, and brazen ages; in allusive reference, an age of wickedness, cruelty, or oppression.

In archaeology, the Iron Age denotes a prehistoric period that followed the Bronze Age, when weapons and tools came to be made of iron. It is conventionally taken as beginning in the early 1st millennium bc, but iron-working began with the Hittites in Anatolia in c.1400 bc. Its arrival in Britain was associated with the first Celtic immigrants in about the 6th century bc. In much of Europe it ended at the Roman period, but outside the Roman Empire it continued to the 4th–6th centuries ad.
Iron Chancellor the nickname of the German statesman Otto von Bismarck (1815–98), Chancellor of the German Empire (1871–90). In recent times it has been applied to the British Chancellor of the Exchequer, Gordon Brown.
Iron Cross the highest German military decoration for bravery, originally awarded in Prussia (instituted 1813) and revived by Hitler in 1939.
Iron Crown of Lombardy the hereditary crown of the ancient kings of Lombardy, so called from having a circlet of iron inserted, reputed to have been made from one of the nails of the Cross.
Iron Curtain a notional barrier separating the former Soviet bloc and the West prior to the decline of communism that followed the political events in eastern Europe in 1989. The phrase is particularly associated with a speech by Winston Churchill in 1946, ‘From Stettin in the Baltic to Trieste in the Adriatic an iron curtain has descended across the Continent,’ although the term in relation to the Soviet Union and her sphere of influence is recorded intermittently from 1920.
Iron Duke a nickname of the Duke of Wellington (1769–1852), recorded from the mid 19th century.
the iron entered into someone's soul someone has become deeply and permanently affected by imprisonment or ill-treatment. It comes from the Latin ferrum pertransit animam ejus, a mistranslation in the Vulgate of the Hebrew, literally ‘his person entered into the iron’, i.e., he was placed in chains or fetters.
iron hand in a velvet glove ruthlessness disguised by courtesy; recorded in Carlyle's Latter-day Pamphlets (1850) as defined by Napoleon.
Iron Lady the nickname of Margaret Thatcher (1925– ), given her in January 1976 by the Soviet defence ministry newspaper Red Star, which accused her of trying to revive the Cold War.
iron mask that worn by the Man in the Iron Mask, a political prisoner in France at the time of Louis XIV, said by some to be a brother of the king, who was made to wear a mask supposedly of iron; he died in the Bastille in 1703, and his identity is still disputed.
iron triangle a grouping of three power bases for mutual defence and support, as for example the Pentagon, the defence industry, and Congress.

See also blood and iron, rule with a rod of iron, strike while the iron is hot.

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melting point: 1,535°C
boiling point: 2,750°C
density: 7.874 g/cm 3(at 20°C)
most common ions: Fe2+ , Fe 3+

Iron, believed to have been introduced on Earth by meteors, was found in Egyptian tombs dating from 3500 b.c.e. The Hittites (in the area known today as Turkey) smelted iron from ore around 1500 b.c.e. From ancient times to the present, the major use of iron has been in the production of steel.

Elemental iron, the major element in Earth's core, is the fourth most abundant element in Earth's crust (about 5.0% by mass overall, 0.5%5% in soils, and approximately 2.5 parts per billion in seawater.) In the crust, iron is found mainly as the oxide minerals hematite, Fe2O3, and magnetite, Fe3O4. Other common mineral forms are siderite, FeCO3, and various forms of FeO(OH). Iron is an essential element in almost all living organisms. In the human body, its concentration ranges between 3 and 380 parts per million (ppm) in bone, 380450 ppm in blood, and 201,400 ppm in tissue.

Iron has a very stable nucleus and has fourteen known isotopes . Four isotopes, 54Fe (5.9%), 56Fe (91.72%), 57Fe (2.1%), and 58Fe (0.28%) make up essentially 100 percent of naturally occurring iron. Pure iron is a soft, white, lustrous metal . Elemental iron oxidizes in moist air but is stable in dry air. Finely divided elemental iron is pyrophoric. Iron dissolves in dilute mineral acid and in hot sodium hydroxide solution. Iron has seven oxidation states (2, 0, +1, +2, +3, +4, and +6) with the +2, ferrous or Fe(II), and +3, ferric or Fe(III), states being the most common. With mild heating, iron reacts with the halogens and with sulfur, phosphorus, boron, carbon, and silicon to form a variety of compounds.

see also Hemoglobin; Industrial chemistry, Inorganic.

Douglas Cameron


Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; and Bochmann, Manfred (1999). Advanced Inorganic Chemistry: A Comprehensive Text, 6th edition. New York: Wiley.

Emsley, John (2001). Nature's Building Blocks: An AZ Guide to the Elements. New York: Oxford University Press.

Lide, David R., ed. (1991). The CRC Handbook of Chemistry and Physics, 71st edition. Boca Raton, FL: CRC Press.

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An ancient observation on the occult virtues of iron was made by Pliny the Elder (ca. 23-79 C.E.) in his Natural History (as translated in 1601 by Philemon Holland).

"As touching the use of Yron and steele in Physicke, it serveth otherwise than for to launce, cut and dismember withal; for take the knife or dagger, an make an ymaginerie circle two or three times round with the point thereof upon a young child or an elder bodie, and then goe round withall about the partie as often, it is a singular preservative against all poysons, sorceries, or enchantments. Also to take any yron naile out of the coffin or sepulchre wherein man or woman lieth buried, and to sticke the same fast to the lintle or side post of a dore, leading either to the house or bed-chamber where any dooth lie who is haunted with Spirits in the night, he or she shall be delivered and secured from such phanasticall illusions. Moreover, it is said, that if one be lightly pricked with the point of sword or dagger, which hath been the death of a man, it is an excellent remedy against the pains of sides or breast, which come with sudden prickes or stitches."

In certain parts of Scotland and Ireland, there was a belief in the potency of iron for warding off the attacks of fairies. An iron poker, laid across a cradle, would, it was believed, keep fairies away until the child was baptized. The Reverend John G. Campbell in his Superstitions of the Highlands and Islands of Scotland (1900) relates how, when children, he and another boy were believed to be protected from a fairy that had been seen at a certain spot because one boy possessed a knife and the other a nail.

Many other countries had folklore about iron as a religious taboo or a charm against witchcraft and the supernatural. Iron tools were prohibited in Greek and Hebrew temples in ancient times. In Korea the body of the king was never to be touched by iron. Roman priests were forbidden to shave with iron blades. In India and China evil spirits were warded off by iron.


French, Roger. Science in the Early Roman Empire: Pliny the Elder, His Sources and His Influence. New York: Barnes & Nobel, 1986.

Pliny the Elder. Natural History. New York: Penguin, 1991.

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i·ron / ˈīərn/ • n. 1. a strong, hard magnetic silvery-gray metal, the chemical element of atomic number 26, much used as a material for construction and manufacturing, esp. in the form of steel. (Symbol: Fe) ∎  compounds of this metal, esp. as a component of the diet: serve liver as it's a good source of iron | [as adj.] how are your iron levels? ∎  used figuratively as a symbol or type of firmness, strength, or resistance: her father had a will of iron | [as adj.] the iron grip of religion on minority cultures. 2. a tool or implement now or originally made of iron: a caulking iron. ∎  (irons) fetters or handcuffs. ∎ inf. a handgun. 3. a hand-held implement with a flat steel base that is heated (typically with electricity) to smooth clothes, sheets, etc. 4. a golf club with a metal head (typically with a numeral indicating the degree to which the head is angled in order to loft the ball). 5. Astron. (also iron meteorite) a meteorite containing a high proportion of iron. • v. [tr.] smooth (clothes, sheets, etc.) with an iron. PHRASES: have many (or other) irons in the fire have many (or a range of) options or courses of action available or be involved in many activities or commitments at the same time. in irons 1. having the feet or hands fettered. 2. (of a sailing vessel) stalled head to wind and unable to come about or tack either way. iron hand (or fist) used to refer to firmness or ruthlessness of attitude or behavior: Fascism's iron hand. an iron hand (or fist) in a velvet glove firmness or ruthlessness cloaked in outward gentleness.PHRASAL VERBS: iron something out remove creases from clothes, sheets, etc., by ironing. ∎ fig. solve or settle difficulties or problems: they had ironed out their differences. DERIVATIVES: i·ron·er n. i·ron·like / -ˌlīk/ adj.

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iron An essential mineral. The average adult contains 4–5 g of iron, of which 60–70% is present in the blood as haem in the circulating haemoglobin, and the remainder present in myoglobin in muscles, a variety of enzymes, and tissue stores. Iron is stored in the liver as ferritin, in other tissues as haemosiderin, and as the blood transport protein transferrin.

Iron balance: losses in faeces 0.3–0.5 mg per day, in sweat and skin cells 0.5 mg, traces in hair and urine, total loss 0.5–1.5 mg per day. Blood loss leads to a considerable loss of iron. The average diet contains 10–15 mg, of which 0.5–1.5 mg is absorbed. The haem iron of meat and fish is considerably better absorbed than the inorganic iron of vegetable foods. Reference intakes are 8.7 mg for adult men and 14.8 mg for women; women who have heavy menstrual blood losses may not be able to obtain enough from food, and supplements are necessary.

Absorption of iron is aided by vitamin C taken at the same time as iron‐containing foods, and reduced by calcium, phosphate and phytic acid. Iron content of foods per 100 g: liver 6–14 mg, cereals up to 9 mg, nuts 1–5 mg, eggs 2–3 mg, meat 2–4 mg. Iron is added to flour so that it contains not less than 1.65 mg per 100 g. Fortified cereals provide 35% of the iron of British diets. Prolonged deficiency gives rise to anaemia.

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Iron is a vital component of heme, the component of hemoglobin that transports oxygen in the blood. Iron deficiency is the world's most common cause of anemia (blood with low hemoglobin and red blood cell components). While some plants have modest amounts of iron (e.g., spinach), meat (red or white) has many times more iron than plants. Meat iron is also absorbed much more efficiently than plant iron. In addition to oxygen transport, iron and heme are key to normal brain development. Iron deficiency during the first six months of life can irreversibly impair cognitive development.

Kenneth R. Bridges

(see also: Hematocrit; Hemoglobin )


Bridges, K. R. (2000). "Iron Deficiency." In Coun's Current Therapy, ed. R. E. Rakel. Philadelphia, PA: W. B. Saunders Company.

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"Iron." Encyclopedia of Public Health. . 17 Aug. 2017 <>.

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iron (symbol Fe) Common metallic element of the first transition series, known from the earliest times. Its chief ores are hematite (Fe2O3), magnetite (Fe3O4), and iron pyrites (FeS2). Pig iron is made in a blast furnace by smelting iron oxide with carbon monoxide from coke, using limestone to form a slag. Cast iron is made from pig iron by remelting and cooling. Wrought iron is made from pig iron by heating with ferric oxide. Iron corrodes to form rust. Most iron is alloyed with carbon and other elements in the various forms of steel used in cutlery, car parts, bridges, ships, and buildings. Properties: 26; r.a.m. 55.847; r.d. 7.86; m.p. 1535°C (2795°F); b.p. 2750°C (4982°F); most common isotope Fe56 (91.66%).

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"iron." World Encyclopedia. . 17 Aug. 2017 <>.

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iron Symbol Fe. A silvery malleable and ductile metallic element that is the fourth most abundant element in the earth's crust. It is required as a trace element (see essential element) by living organisms. Iron is an important constituent of haemoglobin and the cytochromes, being stored in the liver in the form of ferritin. In animals deficiency of iron results in a form of anaemia.

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"iron." A Dictionary of Biology. . 17 Aug. 2017 <>.

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iron (I-ŏn) n. an element essential to life. The body of an adult contains on average 4 g of iron, over half of which is contained in haemoglobin in the red blood cells. Iron is an essential component in the transfer of oxygen in the body; a deficiency of iron may lead to anaemia. Many preparations of iron are used to treat iron-deficiency anaemia. Symbol: Fe.

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"iron." A Dictionary of Nursing. . 17 Aug. 2017 <>.

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iron (Fe) An element required by plants. It is used in reactions in which rapid oxidation reductions occur by the transfer of electrons, as in photophosphorylation and oxidative phosphorylation. Other roles are not understood. Iron-deficient plants have chlorotic (see CHLOROSIS) young leaves; at first the veins remain green but later they too become chlorotic.

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"iron." A Dictionary of Plant Sciences. . 17 Aug. 2017 <>.

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iron(Fe) An element required by plants. It is used in reactions in which rapid oxidation reductions occur by the transfer of electrons, as in photophosphorylation and oxidative phosphorylation. Other roles are not understood. Iron-deficient plants have chlorotic (see chlorosis) young leaves; at first the veins remain green but later they too become chlorotic.

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"iron." A Dictionary of Ecology. . 17 Aug. 2017 <>.

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iron sb. OE. īren, perh. for *īrern, alt. of īsern (by assoc. with the var. īsen) = OS., OHG. īsarn (Du. ijzer, G. eisen), ON. īsarn, Goth. eisarn :- Gmc. *īsarnam, prob. — Celt. *īsarno- (W. haearn, Ir. iarann).
Hence ironclad cased with iron or steel plates, spec. of ships, XIX. ironmonger XIV.

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"iron." The Concise Oxford Dictionary of English Etymology. . 17 Aug. 2017 <>.

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"iron." The Concise Oxford Dictionary of English Etymology. . Retrieved August 17, 2017 from


ironBrian, cyan, Gaian, Geminian, Hawaiian, ion, iron, Ixion, lion, Lyon, Mayan, Narayan, O'Brien, Orion, Paraguayan, prion, Ryan, scion, Uruguayan, Zion •andiron •gridiron, midiron •dandelion • anion • Bruneian •cation, flatiron •gowan, Palawan, rowen •anthozoan, bryozoan, Goan, hydrozoan, Minoan, protozoan, protozoon, rowan, Samoan, spermatozoon •Ohioan • Chicagoan • Virgoan •Idahoan •doyen, Illinoisan, IroquoianEwan, Labuan, McEwan, McLuhan, Siouan •Saskatchewan • Papuan • Paduan •Nicaraguan • gargantuan •carbon, chlorofluorocarbon, graben, hydrocarbon, Laban, radiocarbon •ebon • Melbourne • Theban •gibbon, ribbon •Brisbane, Lisbon •Tyburn •auburn, Bourbon •Alban • Manitoban • Cuban •stubborn •Durban, exurban, suburban, turban, urban

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"iron." Oxford Dictionary of Rhymes. . 17 Aug. 2017 <>.

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